An Italian magazine has published pictures of a pregnant Duchess of Cambridge wearing a bikini on holiday in the Caribbean, angering the British royal family and reigniting a debate over privacy and freedom of the press.

Photographs of the former Kate Middleton, walking with her husband Prince William on a beach on the island of Mustique are expected to run in the Italian gossip magazine Chi on Wednesday (local time).

The same publication ran pictures last September of the princess sunbathing topless on holiday in France, raising questions about media intrusion into the lives of one of the world's most famous couples.

Chi's owner The Mondadori group published a picture of the magazine's front page on its website. It appeared to show the 31-year-old princess wading into the sea in a blue bikini in one picture and another where she was strolling arm in arm with William, 30, second in line to the British throne.

The headline read: "Kate & William in Mustique: The belly grows".

A St James's Palace spokesman said the decision was "a clear breach of the couple's right to privacy".

"We are disappointed that photographs of the Duke and Duchess on a private holiday look likely to be published overseas," he said.

A source told Us Weekly the photos have become "a real sour note".

The Duchess was said to be mortified by how little control she had over the context of how her baby bump would first be revealed to the masses.

"They thought with the privacy laws on the island they would be able to spend a moment outside of the house without this happening," the source said.

The palace has not confirmed if legal action will be taken.

Chi's publication of the topless pictures last year against the couple's wishes was condemned by British Prime Minister David Cameron. It stirred memories of the world media's relentless pursuit of William's mother, Princess Diana who died in a car crash in Paris in 1997 as she was being chased by paparazzi.

While magazines and websites in Europe have printed long-lens shots of the couple, Britain's tabloid newspapers have so far held back.

The newspapers are fighting stricter regulation recommended by a judge-led inquiry into their tactics that uncovered illegal payments to public officials and phone-hacking to get stories.

The couple are expecting their first baby in July. The duchess was admitted to hospital in December suffering from an acute form of morning sickness.